Field of the Invention
The present invention relates to a fuel cell system for performing high potential avoidance control to improve the durability of a fuel cell stack, and an operating method of the fuel cell system.
Background Art
A fuel cell stack is a power generation system which oxidizes fuel by an electrochemical process to directly convert, into electric energy, energy discharged with the oxidizing reaction. The fuel cell stack has a membrane electrode assembly in which both side surfaces of a polymer electrolyte membrane for selectively transporting hydrogen ions are sandwiched and held by a pair of electrodes made of a porous material. Each of the pair of electrodes has a catalyst layer which comes in contact with the polymer electrolyte membrane, the catalyst layer containing, as a main component, carbon powder which carries a platinum-based metal catalyst, and a gas diffusion layer formed on the surface of the catalyst layer and having both air permeability and electron conductivity.
In a fuel cell vehicle in which a fuel cell system is mounted as a power source, the fuel cell stack is made to generate power in a high power range with good generating efficiency to supply power to a traction motor from both the fuel cell stack and a secondary battery or only from the fuel cell stack. On the other hand, in a low power range with poor generating efficiency, operation control is so performed that the power generation of the fuel cell stack will be temporarily halted to supply power to the traction motor only from the secondary battery. The temporary halting of the operation of the fuel cell stack in a low load range where the power generation efficiency of the fuel cell system is low is called an intermittent operation. In the low load range where the power generation efficiency of the fuel cell system is lowered, the intermittent operation can be performed to operate the fuel cell stack in a range with high energy conversion efficiency, and this can increase the efficiency of the entire fuel cell system.
In the fuel cell system, high potential avoidance control is performed to improve the durability of the fuel cell stack. For example, Patent Document 1 discloses a fuel cell system including a fuel cell that receives the supply of reactant gas to generate power; and a controller for performing high potential avoidance control on the output voltage of the fuel cell by setting high potential avoidance voltage lower than the open end voltage thereof as an upper limit. When part of power generated by the fuel cell is stored in a secondary battery, the fuel cell system performs feedback control on the charging/discharging of the secondary battery in consideration of the amount of power stored in the secondary battery. In this case, when the high potential avoidance control is prohibited, the feedback control is allowed, while when high potential control is allowed, the feedback control is prohibited to avoid error integration of an integral term of the feedback control due to the storage of power in excess of the required power of the secondary battery.